Electrical heating element

ABSTRACT

An electrical heating element material of a sheet form is provided, which is comprised of a web of a synthetic resin having a plurality of metal conductor wires arranged therein in parallel to one another along the longitudinal direction of the web. The web is folded in succession at predetermined intervals along folding lines inclined at a small acute angle to the transverse direction of the web to form a substantially two-layer parallelogrammatic sheet. Considering the small angle, the sheet is, for practical purposes, rectangular. The electrical heating element material is prepared by supplying a plurality of metal conductor wires arranged in parallel to one another into a cross-head type extrusion die, separately supplying a thermoplastic resin into said die, co-extruding the resin together with the wires to form a web, folding the web in succession at predetermined intervals along folding lines inclined at an acute angle to the transverse direction of the web to form the two-layer rectangular sheet, and then, optionally, bonding together the two layers. The electrical heating element material may be used as a heat-shrinkable cover, which is comprised of a heat-shrinkable synthetic resin member and the electrical heating element material combined with the heat-shrinkable resin member substantially over the entire region thereof.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to an electrical heating element material of asheet form which comprises a synthetic resin matrix having metalconductor wires arranged therein and to a process for preparing theelectrical heating element material. It also relates to aheat-shrinkable synthetic resin cover including this electrical heatingelement material therein.

(2) Description of the Prior Art

Tubular or flat sheet-shaped heat-shrinkable synthetic resin covers haveheretofore been used as anti-corrosive coverings of joints of steelpipes for pipelines or as protective coverings of joints of electriccables. More specifically, a tubular heat-shrinkable cover is placedaround the joint, or a flat sheet-shaped heat-shrinkable cover is woundaround the joint and the ends thereof are joined together, and theheat-shrinkable cover is shrunk under heating from an appropriateexternal heat source to form a coating tightly covering the joint.According to the conventional method, shrinkage under heating isaccomplished by direct contact with a flames while using a gas burner orthe like. In this heat-shrinking method, great technical skill isrequired for formation of coatings, and under violent climaticconditions, for example, when it is extremely cold or there is a strongwind, this heat-shrinking operation is very troublesome and difficult,and it is impossible to attain a substantially uniform and sufficientheating effect. Therefore, it is impossible to form a sufficientlyanti-corrosive or protective coating or covering on the above-mentionedjoint.

As means for moderating the above-mentioned defects of the conventionalheat-shrinkable covers of the external heating type, there can beconsidered a method in which an electrical heating element is built inthe interior of a heat-shrinkable cover. In this heat-shrinkable cover,when it is located around the joint and shrunk under heating, sinceexternal heating using a heat source such as a burner or a heat raygenerator need not be carried out, it is expected that heat-shrinking ofthis cover can be accomplished irrespectively of climate conditions.

In the heat-shrinkable cover including the electrical heating element,this heating element is required to heat the entire cover uniformlywhile not substantially degrading the heat shrinkability of the cover.Furthermore, this heating element should be manufactured with ease on anindustrial scale, and it should be possible to place the heating elementin the cover simply and easily to facilitate the manufacture of theheat-shrinkable cover.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide anelectrical heating element material of a sheet form having metalconductor wires arranged therein, the whole of which can be uniformlyheated without substantial influence upon the heat shrinkability.

It is another object of the present invention to provide a process forpreparing the above-mentioned electrical heating element material of asheet-form having metal conductor wires arranged therein, wherein themetal conductor wires can be easily arranged in a synthetic resin matrixof the heating element material.

Other objects and advantages of the present invention will be apparentfrom the following description.

In one aspect of the present invention, there is provided an electricalheating element material of a sheet form comprising a web of a syntheticresin having a plurality of metal conductor wires arranged therein, inparallel to one another, along the longitudinal direction of the web,said web being folded in succession at predetermined intervals alongfolding lines inclined at a small acute angle to the transversedirection of the web to form an two-layer rectangular sheet.

In another aspect of the present invention, there is provided a processfor preparing an electrical heating element material of a sheet formcomprising a web of a synthetic resin having a plurality of metalconductor wires arranged therein, which process comprises supplying aplurality of metal conductor wires arranged in parallel to one anotherinto a cross-head type die for extrusion, separately supplying athermoplastic resin into said die, co-extruding the thermoplastic resintogether with the metal conductor wires to form a web having the metalconductor wires arranged therein, folding the web in succession atpredetermined intervals along folding lines inclined at a small acuteangle to the transverse direction of the web to form an two-layerrectangular sheet, and, optionally, bonding together the two layers ofthe resulting sheet.

In still another aspect of the present invention, there is provided aheat-shrinkable cover comprising a heat-shrinkable synthetic resinmember and an electrical heating element material combined with theheat-shrinkable resin member substantially over the entire regionthereof wherein the electrical heating element material comprises a webof a synthetic resin having a plurality of metal conductor wiresarranged therein in parallel to one another along the longitudinaldirection of the web, said web being folded in succession atpredetermined intervals along folding lines inclined at a small acuteangle to the transverse direction of the web to form an two-layerrectangular sheet, and the direction of the folding lines forming theside edges of the two-layer rectangular sheet of the electrical heatingelement material is substantially in agreement with the direction ofheat shrinkage of the heat-shrinkable synthetic resin member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are top plan and cross-sectional views, respectively,illustrating a part of an example of a long web used for the preparationof the electrical heating element material according to the presentinvention;

FIG. 3 is a schematic diagram illustrating the state where this web isbeing prepared by an extrusion process using a cross-head type extrusiondie;

FIG. 4 is a top plan view showing an embodiment of the electricalheating element material according to the present invention;

FIG. 5 is a perspective view partially schematically showing anembodiment of the sheet-shaped heat-shrinkable cover according to thepresent invention; and,

FIG. 6 is an end view partially schematically illustrating an embodimentof the tubular heat-shrinkable cover according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The electrical heating element material of the present invention can beprepared in a discontinuous or continuous manner by two relativelysimple steps, that is, a step of forming a thermoplastic resin webincluding a plurality of metal conductor wires arranged therein inparallel to one another and a step of folding this web in succession atpredetermined intervals to form a two-layer essentially rectangularsheet. Accordingly, the electrical heating element material of thepresent invention is stable in its characteristics, and can bemanufactured very easily.

Since the electrical heating element material of the present inventionhas an essentially rectangular sheet-like shape as a whole, if thiselectrical heating element material is used, a heat-shrinkable cover canbe prepared very easily by inserting this sheet-shaped heating elementmaterial between layers of a heat-shrinkable synthetic resin member andlaminating the assembly together.

In the electrical heating element material of the present invention,since the web including a plurality of metal conductor wires arrangedtherein in parallel to one another is folded in succession along foldinglines at a small acute angle to the transverse direction of the web toform a two-layer essentially rectangular sheet and a plurality of thewires are arranged in a net-like pattern, the metal conductor wires aredensely distributed uniformly over the entire region of the rectangularsheet. Therefore, uniform heating can be attained when an electriccurrent is passed through this heating element material.

In the electrical heating element material of the present invention,there is not present any conductor wire aligned along either of the sideedges of the heating element material, i.e., along the direction of thefolding lines in the web, and only conductor wires arranged in anet-like pattern are included. Therefore, the electrical heating elementmaterial can be stretched and contracted upon heating in the directionof the folding lines. Furthermore, since the heat-shrinkable syntheticresin member including the electrical heating element material insertedbetween at least two layers of the synthetic resin member is subjectedto lamination in the state where the direction of the side edges, i.e.,the folding lines of the electrical heating element material issubstantially in agreement with the direction of heat shrinkage of theheat-shrinkable synthetic resin member, the heat shrinkability of theresulting heat-shrinkable cover is not degraded by the presence of theelectrical heaing element material.

Moreover, since the electrical heating element of the present inventionis formed of a thermoplastic synthetic resin including a plurality ofmetal conductor wires, the included metal conductor wires are completelycovered and insulated by the synthetic resin. Accordingly, even if twoadjacent wires arranged in parallel to each other are brought very closeto each other in the web, or even when the web is folded so that thewires arranged in parallel to each other form a net-like pattern, shortcircuits or breakages of the metal conductor wires are not caused.Furthermore, even if one conductor wire is broken at a certain point, asufficient heating effect can be attained by other conductor wires.

Since the web of the electrical heating element material of the presentinvention is formed of a thermoplastic synthetic resin including aplurality of metal conductor wires, when the heating element material isinserted between at least two layers of the heat-shrinkable syntheticresin member and the assembly is subjected to lamination, the entireassembly is integrally fusion-bonded very smoothly and a heat-shrinkablecover having no voids in the interior thereof can be prepared.

The electrical heating element material of the present invention, theprocess for the preparation thereof and the heat-shrinkable coverincluding this electrical heating element material therein will now bedescribed in detail with reference to the accompanying drawings.

Referring to FIGS. 1 and 2, illustrating one example of a long web usedfor the manufacture of the electrical heating element material of thepresent invention, a rectangular web 1 is formed of a synthetic resinand it includes therein a plurality of metal conductor wires 2 arrangedin parallel to one another along the longitudinal direction of theweb 1. Any flexible and soft metallic materials which generate heatunder application of electricity, that is, electrically resistantmetallic materials, can be used for the metal conductor wires 2. Singlewires or twisted wires having a fine diameter, which are formed ofcopper, a copper alloy or a nichrome alloy, are preferably used as themetal conductor wires. In the case of copper wires, it is preferred thatthe diameter be 0.01 to 1.0 mm, especially 0.1 to 0.6 mm, and thesectional area be 0.0001 to 0.8 mm², especially 0.001 to 0.3 mm². Thesectional shape of metal conductor wires need not be absolutelycircular.

Any of the thermoplastic resins may be used for formation of the web 1.For example, there can be mentioned polyethylene, polypropylene,polyvinyl chloride, polyesters and polyamides. After the web has beenformed from such material, the web may be irradiated with electronradiation or other radiation or chemically treated with a cross-linkingagent, such as a peroxide compound, to effect cross-linking. It ispreferable that the degree of cross-linking be such that the degree ofgel fraction does not exceed 70%, and more preferably, the degree of gelfraction is in the range of the gelation percentage from about 35% toabout 65%. A heat resistance, i.e., the property of preventing flowand/or deformation at high temperature, can be imparted to the syntheticresin shaped article by this cross-linking.

By the term "degree of gel fraction" used herein is meant a weightpercent of a residual sample based on the weight of the initial sample,as measured when 0.15 g of the initial sample is dipped in 30 ml ofxylene, at 120° C., for 24 hours, and then, the sample is maintained at80° C. for more than 16 hours to remove xylene therefrom.

The web 1 includes therein a plurality of metal conductor wires 2arranged in parallel to one another along the longitudinal direction ofthe web 1. It is preferable that the distance between every two adjacentparallel metal conductor wires in the web be 0.3 to 2 cm, morepreferably 0.5 to 1 cm. If this distance is too large, a uniform heatingeffect cannot be obtained and a sufficient quantity of heat cannot beobtained. On the other hand, if the distance is too small, sinceadjacent metal conductor wires are brought very close to each other andsometimes into contact with each other at the heat-shrinkage step, shortcircuits of the metal conductor wires are sometimes encountered.

Suitable width of the web 1 varies depending upon the particular size ofthe heat-shrinkable cover and thus upon the particular size of a pipejoint to be covered with the heat-shrinkable cover. For example, whenthe web is used for the electrical heating element material to beincluded in a heat-shrinkable cover for a pipe joint having a diameterof about 60 inches (about 152.4 cm), the width of the web mayconveniently be about 5 to about 30 cm. When the web is used for theelectrical heating element material to be included in a heat-shrinkablecover for a pipe joint having a diameter of about 30 inches (about 76.2cm), it is convenient that the width is about 3 to about 15 cm.Furthermore, when the web is used for the electrical heating elementmaterial to be included in a heat-shrinkable cover for a pipe jointhaving a diameter of 12 inches (about 30.5 cm), it is convenient thatthe width of the web is about 1.0 to about 5 cm.

It is preferable that the thickness of a synthetic resin portion 3 (FIG.1 or 2) of the web 1 is 10 to 500 microns, more preferably about 20 toabout 300 microns and that the thickness of the synthetic resin coatingthe metal conductor wire 2 in the web 1 be 10 to 200 microns, morepreferably about 20 to about 100 microns.

The above-mentioned web can be prepared according to various methods.For example, the web may be prepared by an extrusion process using across-head type die 4 for extrusion as illustrated in FIG. 3. In thisregard, referring to FIG. 3, a plurality of metal conductor wires 2arranged in parallel to one another are supplied from the rear of thedie 4, and separately, a molten thermoplastic resin is supplied into acavity 5 of the die 4 from a feed inlet 6. The thermoplastic resin isco-extruded together with the metal conductor wires in the form of a weband cooled to form a web including the metal conductor wires therein.According to this method, a long web including metal conductor wiresarranged therein in parallel to one another along the longitudinaldirection thereof can be advantageously prepared in a continuous mannerwith a stable quantity being guaranteed.

Referring to FIG. 4, the electrical heating element 11 of the presentinvention comprises the above-mentioned rectangular web 1, which isfolded in succession at predetermined intervals along folding lines 13inclined at a small acute angle (see FIG. 1) to the transverse directionof the web 1, to form a substantially two-layer parallelogrammaticsheet. This sheet-shaped electrical heating element 11 comprising thetwo-layer web can be used as it is, but in such case, there is a risk ofthe sheet opening while it is being handled. Accordingly, it ispreferable that the two layers of the web be partially or entirelybonded together. For example, the two layers of the sheet-shapedelectrical heating element 11 are thermally press-bonded directly orthrough a hot-melt adhesive layer (for example, a heat sealer) to forman integral laminate.

The folding lines 13 of the web 1 become the two side edges of thesheet-shaped electrical heating element 11, and it is preferable thatthe direction of the folded lines 13 be inclined at an acute angle of0.5 to 10°, more preferably 1° to 8°, to the transverse direction of theweb, i.e., the direction perpendicular to the longitudinal direction ofthe web. If the inclination angle of the folding lines is too large withrespect to the transverse direction of the web, the heat shrinkabilityof a heat-shrinkable cover formed by including this electrical heatingelement material in a heat-shrinkable synthetic resin member isdegraded.

As pointed out hereinbefore, the folding lines 13 of the web become twoside edges 14 and 15 parallel to each other. Each segment 13' of thefolding lines forming side edges of the two-layer sheet may overlap withthe adjacent segments at either of the two and portions thereof.However, the length of each overlapped portion should preferably be lessthan one cm, more preferably less than 0.5 cm. Furthermore, each segment13' of the folding lines forming side edges of the two-layer sheet maybe separated from the adjacent segments. However, the distance betweenthe two adjacent segments should preferably be less than one cm, morepreferably less than 0.5 cm. It is most preferable that each segment 13'of the folding lines be contiguous to the adjacent segments without anysubstantial space between, as illustrated in FIG. 4. If the adjacentsegments of the folding lines overlap each other with an overlappinglength of more than one cm or they are separated from each other by adistance of more than one cm, a uniform heating effect cannot beobtained by the resulting electrical heating element material.

The electrical heating element 11 of the present invention haselectricity-applying terminals 9 on both ends of the folded web, towhich the respective metal conductor wires are connected. Generally, itis preferable that a pair of the terminals 9 be fitted on both endportions of the folded web having the metal conductor wires, asillustrated in FIG. 4. However, a plurality of terminals may be formedin intermediate portions in addition to the terminals 9 located on bothend portions, if necessary. These terminals may be arranged on one ofthe two side edges i.e., the folding lines of the folded web) of theelectrical heating element material, or the paired terminals may bearranged on opposite side edge lines, respectively.

The electrical heating element material of the present invention can beprepared by folding the web in succession at predetermined intervalsalong folding lines inclined at an acute angle to the transversedirection of the web to form a substantially two-layerparallelogrammatic sheet and, optionally, bonding the two layers of thesheet to each other to form an integral laminate.

As the method for folding the synthetic resin web, there can bementioned, for example, a method in which the web is spirally woundtightly and closely on a strip of plate having a certain width, theplate is then removed and the two layers of the folded web are laminatedby heat bonding under pressure, and a method in which the web is foldedin succession at predetermined intervals in a zigzag manner so thatevery two adjacent folding lines are contiguous to each other and thetwo layers of the folded web are laminated by heat bonding underpressure. This heat bonding under pressure is carried out only forretaining the rectangular sheet-like shape of the folded electricalheating element material. Care should be taken that conductor wires donot intersect one another and cause short circuits because ofinsufficient insulation. A hot melt adhesive or the like may be used forbonding, if desired.

In the electrical heating element of the present invention, the metalconductor wires included in the heating element material are arranged atan acute angle to the direction perpendicular to the direction of thetwo side edges of the heating element material, i.e., the direction ofthe folding lines in the web, as illustrated in FIG. 4. Accordingly,stretching or contraction of the electrical heating element materialincluded in the heat-shrinkable cover is allowed in the direction of theside edges (the direction of the folding lines) at the step of heatingthe heat shrinkable cover. If the direction of the side edges of theheating element is made substantially in agreement with the direction ofheat shrinkage of the heat-shrinkable cover, the heat shrinkability ofthe cover is not degraded at all because of the presence of the includedheating element material.

Furthermore, in the electrical heating element of the present invention,since the metal conductor wires included and arranged in parallel to oneanother cross one another at a great number of points to form a net-likepattern as illustrated in FIG. 4, a uniform heating effect can beattained.

Moreover, since an electric current can be supplied in parallel throughthese metal conductor wires arranged in parallel to one another, a largequantity of heat can be generated, and even if wire breakage takes placein one conductor wire, heating can be accomplished by the otherconductor wires.

In the heat-shrinkable cover of the present invention, theabove-mentioned electrical heating element is included substantiallyover the entire region of the heat-shrinkable synthetic resin member sothat the direction of the side edges of the folded web of the heatingelement is substantially in agreement with the direction of heatshrinkage of the heat-shrinkable synthetic resin member.

As the heat-shrinkable synthetic resin member, there may be employed afilm or sheet material of a crystalline synthetic resin having aheat-shrinkability, or a laminate formed of a plurality of such film orsheet materials. In order to improve the heat resistance, it ispreferable that the film or sheet material be cross-linked by electronradiation, or other radiation so that the degree of gel fraction is 20to 80%, more preferably 30 to 70%. As the crystalline synthetic resin,there can be mentioned, for example, polyethylene, polypropylene,polyvinyl chloride and polyamides.

The heat-shrinkable film or sheet material can be prepared by shaping acrystalline synthetic resin such as mentioned above into a film orsheet, uniaxially stretching the film or sheet at a high temperature(higher than 100° C.) and cooling the film or sheet in the stretchedstate to ambient temperatures. The cross-linking may be carried outbefore or after the stretching. The so-prepared material exhibits a heatshrinkability along the stretching direction, and this stretchingdirection corresponds substantially to the direction of heat shrinkage.

A heat-shrinkable synthetic resin member prepared by shaping a film orsheet from polyethylene, cross-linking and stretching the polyethylenefilm and laminating a plurality, especially 2 to 20, of such films orsheets of cross-linked polyethylene is preferably used for thepreparation of the heat-shrinkable cover of the present invention.

The heat-shrinkable cover of the present invention can be prepared byinserting the electrical heating element 11 between at least two layers16 and 17 and laminated together for formation of the heat-shrinkablesynthetic resin member 10, so that the heating element material isdisposed in a predetermined direction, and heat-bonding the assemblyunder pressure to form a laminate structure.

More specifically, as the method for preparing the heat-shrinkable coverof the present invention, there can be mentioned a method in which theabove-mentioned synthetic resin material of a film or sheet form iswound on a drum in 2 to 10 layers, especially 2 to 6 layers, theelectrically heating element material is wound on the layers of the filmor sheet so that the direction of the folding lines of the heatingelement material is substantially in agreement with the direction ofheat shrinkage of the film or sheet, 2 to 15 layers, especially 2 to 10layers, of the film or sheet are further wound on the heating elementmaterial, a release paper or the like is wound on the outermost layer ofthe film or sheet and the entire assembly is heated to effect laminationwhile the ends of the wound assembly are clamped together, whereby therecan be formed a heat-shrinkable cover comprising a heat-shrinkablesynthetic resin member having the heating element material included inthe interior thereof. The heat-shrinkable cover prepared according tothis method has a tubular shape 12 as illustrated in FIG. 6. When thistubular product is cut between the terminals 9 in FIG. 6, along thedirection parallel to the rotation axis of the drum, a flat sheet-shapedheat-shrinkable cover can be obtained. In the above-mentionedpreparation method, if a release paper is interposed in a portioncorresponding to the above-mentioned cutting portion at the step ofwinding the electrical heating element material, by cutting the tubularheat-shrinkable cover at the release paper-attached portion, asheet-like heat-shrinkable cover 10 having both ends including branchedtongue-like pieces as illustrated in FIG. 5 can be prepared. Thisheat-shrinkable cover having such tongue-like pieces on both ends ispreferred because, when the cover is wound on the joint of a steel pipe,both ends of the cover can be advantageously connected by meshing thetongue-like pieces with each other.

As another method for preparing the heat-shrinkable cover of the presentinvention, there can be mentioned a method in which 2 to 10, especially2 to 6, rectangular films or sheets of the heat-shrinkable material,which are the same in the size, are superposed on a flat plate, theelectrical heating element material of the present invention issuperposed on the films or sheets so that the direction of the foldinglines is substantially in agreement with the direction of heat shrinkagein the films or sheets, films or sheets of the heat-shrinkable materialare further superposed on the heating element material and the assemblyis laminated together by heating while a pressing force is applied tothe entire surface of the assembly in such a manner that the heatshrinkability of the heat-shrinkable material is not lost. According tothis preparation method, a flat sheet-shaped heat-shrinkable cover canbe directly obtained. If a release paper is interposed between thelayers of the heat-shrinkable material as in the above-mentioned methodusing the winding drum, a heat-shrinkable cover 10 having tongue-likepieces on both ends as illustrated in FIG. 5 can be obtained.

As still another method for the preparation of the heat-shrinkable coverof the present invention, there can be mentioned a method in which 2 to20 films, especially 2 to 10 films, of a heat-shrinkable resin materialare superposed upon each other and laminated together to form aheat-shrinkable synthetic resin material, two sheets of such laminatedfilms are prepared, the electrical heating element material is insertedbetween the two sheets and the assembly is heated under pressing to forma laminate structure.

The size of the heat-shrinkable cover of the present invention can bedetermined appropriately according to the diameter of the joint of asteel pipe or electric cable to be covered, but it is preferable thatthe thickness of the cover be 0.5 to 5 mm, more preferably about 1 toabout 3 mm.

Since the heat-shrinkable cover of the present invention includestherein an electrical heating element material having metal conductorwires arranged in a net-like pattern, it is excellent from the point ofview that shrinkage under heating can be accomplished very convenientlyand advantageously. Accordingly, even if a pipeline is constructed undersevere climatic conditions, for example, in cold temperatures or whenthere is a strong wind, a uniform heating effect can be attained bywinding the heat-shrinkable cover of the present invention around theperiphery of the joint of a steel pipeline and passing an electriccurrent through the electrical heating element material included in theheat-shrinkable cover.

The present invention will now be further clarified by the followingExamples.

EXAMPLE 1

Eight copper wires, each having a diameter of 0.26 mm, were supplied toa cross-head type die of an extruder having a screw diameter of 30 mmwhile they were arranged in parallel to one another at intervals of 7.8mm. Separately, high-pressure polyethylene, containing 2.6% by weight ofcarbon black, and having a density of 0.935 and a melt index (MI) of0.12, was supplied to the extruder. The polyethylene was coextruded fromthe cross-head type die, at an extrusion temperature of about 220° C.,together with the copper wires. The extrudate was dipped in a water bathmaintained at ambient temperature to prepare a web having a width of 6.2cm and including therein 8 copper wires arranged in parallel to oneanother. Thereafter, the web was irradiated with electron rays until thedegree of gel fraction reached about 50%.

The web was folded 60 times in succession and the folded web was bondedunder heating to obtain an electrical heating element material having aparallelogrammatic sheet shape 600 mm in width and 187 cm in length. Inthis electrical heating element material, the direction of the foldinglines in the web was inclined at an acute angle of 3.12° to thetransverse direction of the web. Considering the small angle, the sheetcould be considered, for practical purposes, to be rectangular. Therespective segments of the folding lines were close to one another sothat the distance between two adjacent segments was not larger than 1mm.

Terminals were attached to all of the eight copper wires at both endportions of the web, respectively.

Eight films of polyethylene (having a gelation percentage of 48% and aheat shrinkability of 40%) having a thickness of 0.15 mm, a width of 600mm and a length of 186 mm were placed below the heating elementmaterial, and eight of the same polyethylene films were placed above theheating element material. The assembly was heated at 175° C., for 45minutes, under pressing, to fusion-bond the respective layers and effectlamination, whereby a heat-shrinkable cover was obtained.

The heat-shrinkable cover was wound around the periphery of a steel pipepoint having a diameter of 50 cm, and the terminals were connected to analternating current power source of a constant voltage of 110 V and anelectric current was passed through the electrical heating elementmaterial in an ambient temperature of 20° C., to effect heat shrinkage.The heat shrinkage was accomplished within about 3 minutes and ananti-corrosive cover uniformly and closely adhered to the periphery ofthe joint of the steel pipe was obtained.

EXAMPLE 2

A web having a width of 12.5 cm and including 16 copper wires 0.26 mm indiameter, which were arranged in parallel to one another at intervals of7.8 mm, was prepared by extrusion and electron radiation curing in thesame manner as described in Example 1. The web was folded in succession80 times, and an electrical heating element material having a width of850 mm and a length of 499 cm was prepared in the same manner asdescribed in Example 1.

By using this heating element material, a heat-shrinkable cover wasprepared in the same manner as described in Example 1, except that filmsof polyethylene (having a gelation percentage of 48% and a heatshrinkability of 40%) having a thickness of 0.15 cm, a width of 850 mmand a length of 499 cm were used. The so-obtained heat-shrinkable coverwas wound on the periphery of a steel pipe joint having a diameter of155 cm, and an alternating current of 160 V was supplied to theterminals of the cover in an ambient temperature of 20° C. to effectheat shrinkage. The heat shrinkage was accomplished within about 6minutes, and an anti-corrosive cover adhering uniformly and closely tothe periphery of the joint of the steel pipe was formed.

We claim:
 1. An electrical heating element of a sheet form comprising arectangular web of a thermoplastic resin having a plurality of flexiblemetal conductor wires arranged therein in parallel to one another alongthe longitudinal direction of the web, said web being folded insuccession at predetermined intervals along folding lines inclined at asmall acute angle to the transverse direction of the web to form anessentially two-layer rectangular sheet.
 2. An electrical heatingelement according to claim 1, wherein each of the flexible conductorwires has a sectional area of from 0.0001 to 0.8 mm² and the distancebetween every two adjacent parallel metal conductor wires in the web isfrom 0.3 to 2 cm.
 3. An electrical heating element according to claim 1,wherein the thermoplastic resin of the web has a thickness of from 10 to500 microns at the portions where no metal conductor wires exist and athickness of from 10 to 200 microns at the portions where the flexiblemetal conductor wires are coated with the thermoplastic resin.
 4. Anelectrical heating element according to claim 1, wherein the foldinglines are inclined at an acute angle of 0.5 to 10° to the transversedirection of the web.
 5. An electrical heating element according toclaim 1, wherein adjoining edges of adjacent segments bounded by thefolding lines forming the side edges of the two-layer rectangular sheetare in essentially an abutting relationship with overlaps of theadjacent segments being no more than one centimeter, and separation ofthe adjacent segments being no more than one centimeter.
 6. Anelectrical heating element according to claim 1, wherein the two layersof the essentially rectangular sheet are bonded together.
 7. Anelectrical heating element according to claim 1, wherein thethermoplastic resin is cross-linked to such an extent that the degree ofgel fraction is in the range of from about 35% to about 65%.
 8. Anelectrical heating element according to claim 1, wherein a pair ofterminals are fitted on both end portions of the folded web having theflexible metal conductor wires.